This invention relates to dispersions of particles in a liquid vehicle and, more particularly, to dispersions in which the particles are entangled in a cross-linked polymer matrix.
Dispersions of particles in a liquid vehicle are commonly used in a wide variety of industries and processes, such as coatings (e.g., paint and ink), magnetic or optical recording media (e.g., tapes and disks), cosmetics (e.g., lipsticks and nail polish); agriculture (e.g., insecticides), pharmaceutical preparations and many others. In addition, in a concentrated form (such as can be obtained by centrifugation followed by decanting the supernatant liquid) the dispersions are useful for tinting, coloring fibers, coloring molded resins, for adding pigment to flexographic plates and a variety of other applications.
As expected, these dispersions are very diverse. Generally speaking, however, these dispersions all contain a liquid vehicle (such as water, an organic solvent, or a combination of the two) and some type of particle (such as a pigment, a pharmaceutically active compound, metallic flakes, hollow glass spheres, discrete polymer particles, etc.). Typically, but not always, a dispersant is used to help maintain the particles in a suspended state in the liquid vehicle; i.e., prevent the particles from settling out of the liquid. In many instances, the dispersant used is a polymer.
There has been significant effort in the art directed at improving the stability of the dispersions so that the particles are less likely to settle out of the liquid under a defined set of conditions. The reason for the effort is that a dispersion with improved stability can translate into products having a longer shelf life; products that can survive more rigorous storage conditions (e.g., extreme temperature cycles); products that are easier or less expensive to transport or handle during use; and products that are more uniform and consistent in quality, and products that offer greater formulation latitude.
The effort to improve dispersion stability to date have included improvements in the processes used to make the dispersions, the development of new dispersants and the exploration of the interaction between dispersants and particular liquid vehicle formulations. Recently, there has been a good deal of research directed at modifying the particles, especially the particle surface, in order to improve the dispersion stability. For instance, recent advances in the art have seen the advent of coated particles, particles whose surfaces have been chemically modified, and particles that are covalently bonded to a dispersant.
While much of the effort has had general application at improving dispersion stability, some of that effort has not found utility in particular applications. For example, pigment dispersions used in ink jet printing applications have very unique and demanding requirements. Ink jet printing is a non-impact and non-contact printing process in which an electronic signal produces droplets of ink that are deposited on a wide variety of substrates such as paper, transparent film, plastics, metals and fabrics. Typically, the ink is ejected from a printhead containing a plurality of very small nozzles using thermal or piezoelectric ejection technology. In ink jet printing, it is critical that the ink components remain stable, not only in storage but also over repeated firing cycles; that they not interact with the components used to manufacture the printhead; that they not clog the nozzle openings; and that they not for a film on the orifice plate or resistors used in the printhead. In addition, because such printing is often used in an office environment, such inks tend to be aqueous based dispersions.